The manufacture of semiconductors and integrated circuits necessarily involves selective modification of the electrical and physical properties of the substrate from which the devices are to be made. This selective modification is typically accomplished by exposing the substrate to some suitable modifying agent through the windows of an apertured process mask. For example, in the case of silicon, the process mask might comprise a thin, patterned layer of silicon dioxide which has been overlayed on a silicon substrate and the modifying agent might comprise a beam of ions, for example boron.
Advantageously, the process mask is formed in situ, for example, by overlaying the masking material with a layer of some suitable photoresist which is then exposed to the image of the desired mask pattern, using either contact or projection printing. Alternatively, a non-photosensitive resist may be used which is then patterned by electron beam or X-ray lithography. In either event, after exposure, the resist is developed and the exposed portions chemically removed to yield the desired masking pattern.
The next step in the process is generally to remove the underlying masking material, e.g., by a wet chemical etch, through the apertures in the photomask, thereby to form the process mask which is needed for subsequent modification of the underlying silicon substrate.
As is well known, in recent years, there has been an ever-increasing trend towards large scale integration (LSI) and the manufacture of such diverse devices as 16-bit microprocessors, 64K memory chips, etc. Because of the component density in such devices, the line widths which are required to fabricate and to interconnect the active devices on each LSI chip approach .mu.m and sub-.mu.m dimensions.
As a result of these requirements, the use of wet chemical etching to transfer the resist pattern to the underlying device material has become undesirable. The reason for this is that wet chemical etching is an isotropic process; that is, the etch rate is the same in all directions. This means that the etchant tends to undercut the resist at the resist-semiconductor interfaces. The practical effect of this undercutting is that the line widths formed in the process mask are substantially wider than those which were contained in the overlaying resist mask.
To a certain extent, this effect can be compensated for by making the line widths on the photomask substantially narrower than is actually desired for the corresponding line widths on the process mask. This assumes, of course, that the undercutting that occurs during the chemical etch will widen these lines to their desired width. However, since the degree of undercutting that occurs in a given process is difficult to control, this is not an entirely satisfactory solution to the problem. Furthermore, if the line widths on the process mask are such as to push the limits of technology, then obviously to lay down even narrower line widths on the photomask becomes an exceedingly difficult task. This difficulty is compounded by the fact that physical phenomena, such as the wave-length of the light used to transfer the image, may also become significant and limiting factors.